AbstractThe Westerlund 1 Galactic cluster hosts an eclectic mix of coeval massive stars. At a modest distance of 4–5 kpc, it offers a unique opportunity to study the resolved stellar content of a young (~5 Myr) high mass (5·10⁴M_⊙) star cluster. With the aim of testing single-star evolutionary predictions, and revealing any signatures of binary evolution, we discuss on-going analyses of NTT/SOFI near-IR spectroscopy of Wolf-Rayet stars in Westerlund 1. We find that late WN stars are H-poor compared to their counterparts in the Milky Way field, and nearly all are less luminous than predicted by single-star Geneva isochrones at the age of Westerlund 1.